Inkjet printers have printheads that operate a plurality of inkjets to eject liquid ink onto an image receiving member. The ink may be stored in reservoirs located within cartridges installed in the printer. Various forms of ink include aqueous, oil, solvent-based, UV curable inks, or ink emulsions. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink may be pellets, ink sticks, granules, pastilles, or other forms. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. In other inkjet printers, ink may be supplied in a gel form. The gel is also heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
A typical full width inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be a continuous web of recording media, a series of media sheets, or the image receiving member may be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force generated in a transfix nip that is formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink from a pressure chamber through an orifice in response to an electrical signal, also referred to as a firing signal. The amplitudes, or voltage levels, of the signals affect the amount of ink ejected in each drop. The firing signal is generated by a printhead controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
In order for the colors of printed images to correspond closely to the image data, the ink drops ejected onto the media for each ink color should form uniform colors for a given density of the color as specified in the image data. For example, if a region of a media sheet includes a region where 50% of the surface of the sheet should be covered in yellow ink, then the resulting ink image should appear to have a uniform yellow color in the specified region. To achieve the uniform color, the average sizes and masses of individual ink drops that form the ink image should be substantially uniform. In practical embodiments, the ink ejectors in an individual printhead may eject ink drops of various sizes leading to non-uniform colors in the ink images. Additionally, variances in the drop masses of ink drops ejected by different printheads also result in undesirable non-uniformity in color reproduction. One example of artifacts that occur in printed images where the drop masses of different printheads are not uniform are “lawn mower tracks.” This term refers to streaks of color from one printhead that appear to be darker than streaks of color from another printhead. To reduce or eliminate the image artifacts, the ejectors in each printhead and multiple printheads are calibrated to enable the printheads to eject ink drops with uniform masses.
Existing techniques for calibrating printheads to eject ink drops with uniform masses are carried out in an offline manner, such as during the manufacturing process of the printhead or in a specially configured calibration device. The offline calibration process is time consuming and requires that printheads be removed from an imaging device to undergo the calibration procedure. During printing operations, the drop mass of ink drops ejected by a printhead may change due to changes in the operational parameters of the printhead caused by use of the printhead over time. The printhead must then be removed from the imaging device for another calibration process. A procedure for calibrating the printheads in situ where one or more printheads are calibrated to eject ink drops with uniform masses while the printheads are operatively configured in the imaging device would be beneficial.